Controlled synthesis of uniform palladium nanoparticles on novel micro-porous carbon as a recyclable heterogeneous catalyst for the Heck reaction

Article abstract of DOI:10.1039/c5dt02133b

Novel dual-porous carbon-supported palladium nanoparticle (Pd NP) catalysts were prepared by sequential carbonization and reduction of microporous organic polymer-encaged PdCl₂. The diverse pore structure of microporous organic polymers provides a reservoir for the palladium precursors and prevents Pd NPs from sintering during the carbonization and reaction. The microporous structure has a significant effect on the size and dispersion of palladium NPs. The average size of the Pd NPs (in the range of 4-6 nm) was tuned by changing the pore size distribution and the carbonization temperature. The resulting carbon-supported Pd NPs were characterized by TEM, BET, XRD, and XPS and the Pd loading was calculated by AAS. The encaged Pd NP catalysts prepared by this methodology exhibited outstanding stability and reusability in the Heck reaction and could be reused at least 10 times without appreciable loss of activity.

Full text of DOI:10.1039/c5dt02133b

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Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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Controlled synthesis of uniform palladium nanoparticles on novel
micro-porous carbon as a recyclable heterogeneous catalyst for
Heck reaction
Kunpeng Songa, Peng Liua, Jingyu Wanga, Lei Pangd, Jian Chena, Irshad Hussainc, Bien Tanb* and
Tao Lia*
Novel dual-porous carbon-supported palladium nanoparticle (Pd NP) catalysts were prepared by sequential
carbonization and reduction of microporous organic polymer-encaged PdCl2. The diverse pore structure of
microporous organic polymers provides a reservoir for the palladium precursors and prevents Pd NPs from
sintering during the carbonization and reaction. The microporous structure has a significant effect on the size and
dispersion of palladium NPs. The average size of the Pd NPs (in the range of 4-6 nm) was tuned by changing the
pore size distribution and the carbonization temperature. The resulting carbon-supported Pd NPs were
characterized by TEM, BET, XRD, XPS and the Pd loading was calculated by AAS. The encaged Pd NP catalysts
prepared by this methodology exhibited outstanding stability and reusability in Heck reaction which could be
reused at least 10 times without appreciable loss of activity
.
received increasing attention in the last decade due to the
enormous synthetic potential to generate new carbon-carbon
bonds.3 Recent studies are attempted to immobilize Palladium
NPs on various supports such as carbon,4 magnetic materials,5
silica,6 hydroxyapatite,7 zeolites,8 MOFs9 and organic
polymers10 to create heterogeneous catalysts system. Among
these supports, activated carbons have been well addressed
due to numerous promising applications in heterogeneous
catalysis. Carbon has a highly disorganized structure of
aromatic sheets and strips, high surface area and offers
various active sites due to the presence of several functional
groups within its pores.11 The cost-effectiveness, easier
availability, appropriate thermal and mechanical properties
also make carbon based supports more attractive for
heterogeneous catalysis. There are several reports to prepare
carbon supported metal catalysts, including palladium, but
they often suffer from substantial metal/Pd leaching (e.g. 14 %
of Pd is leached from Pd/C) 12 and the aggregation of metal/Pd
NPs at high temperature. So the major limitation of Pd/C
catalysts, whether they are commercially available or
generated in situ, is their poor recyclability.
Introduction
Palladium catalysts play an important role in organic
syntheses, chemical and pharmaceutical process. Most
applications in these processes involve homogeneous catalysts
consisting of palladium complexes with a variety of ligands.1
However, homogeneous catalysis has certain limitations
including the separation, recovery and regeneration of the
catalyst that are the key factors to make the processes cost-
effective. A series of attempts have been made to overcome
these problems by using heterogeneous Palladium catalyst
systems. Because the metal size and dispersion have an
essential impact on the catalysts activity, many researchers
have concentrate on the method of preparation high dispersed
and excellent catalytic efficiency heterogeneous palladium
catalysts.2
The palladium-catalysed Heck coupling reaction has
Therefore, the synthesis of palladium nanoparticles (NPs)
immobilized on porous supports has attracted extensive
interest in heterogeneous catalysis.13 The catalytic activity of
palladium NPs is known to depend on their size and degree of
dispersion. Much effort has been devoted to embedding
palladium NPs into porous materials containing coordination
groups, which can offer potential for the control of particles
size and prevention of NPs aggregation.14 So the controlled
synthesis of catalytically active Pd NPs with stable and narrow
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carbonization in the presence of palladium precursors and
their catalytic applications were evaluated for Heck coupling
reactions. The palladium NPs size effect and the optimization
of the reaction parameters are addressed, aiming at
developing C-(KTB-Pd) as a heterogeneous catalyst with high
size distribution has become the research focus of the leading
researchers in catalysis.15
We report the design and synthesis of a
“dual-porous”
carbon material with the possibility to replace conventionally
used active carbons. Ordered mesoporous carbons (OMCs) are
generally prepared with a hard-templating method relying on
mesoporous silica,16 porous coordination polymers (PCPs) or
metal-organic frameworks (MOFs), which are crystalline
compounds consisting of metal ions coordinated to the rigid
organic framework to form one-, two-, or three-dimensional
structures.17 A new approach is also demonstrated to prepare
novel microporous carbon materials by the carbonization of
microporous organic polymers (MOPs) followed by the
immobilisation of Pd NPs in pore structure, combining the
advantages of MOPs with micro-sized supporting entities that
resulted in a highly active and recoverable catalyst. 18 Recently
various strategies are being developed to produce diverse
classes of porous carbon materials. Among these porous
materials, MOPs with extraordinary high surface area have
attracted extensive attention due to their unique properties
such as large surface area, low skeletal density, and high
chemical stability. One particular advantage of MOPs is to
allow the fine dispersion and stabilization of small metallic
particles. There are, however, only a few reports on MOPs
supported metal nanoparticles with tunable metal NPs size
distribution, especially during the process of carbonization.
Compared to other porous materials, MOPs have the narrow
pore size distribution and complicated pore structures, which
prevents the metal particles sintering during the high-
temperature annealing.
catalytic activity and selectivity for the Heck coupling reaction.
Owing to the large surface area, high microporsity and
excellent rigid network properties of KTB, it is an excellent
supporting material for Pd NPs. Additionally, the control over
the size of Pd NPs and their subsequent effect on their
catalytic activity, and the recycling potential of the carbon-
supported Pd NPs have all been assessed.
It has been observed that the leached palladium species
from the supported catalysts are the real active species in the
Heck reaction, which can be re-deposited onto the surface of
the support when the aryl halides are depleted.19 However,
the palladium species cannot be re-deposited completely after
reaction. One potential strategy to design the stable
heterogeneous catalyst for the Heck reaction is to encapsulate
the palladium species by using the support as a reservoir to
restrict the palladium active species. In this regard, dual-
porous materials with diverse pore distribution and a certain
rigid network structure would be desirable supports, in which
the micro pores can form an effective scaffold for the
deposition of metal nanoparticles, whereas the rigid structure
can protect the collapse of network and restrict the Pd species
to prevent their sintering. More interestingly, the Pd NPs
encaged in the micro network have an excellent stability
compared to the homogeneous and other heterogeneous
catalysts.
Scheme 1. Synthetic route of microporous carbon supported
Pd NPs catalysts.
Experimental Section
Materials: Benzene, FeCl3 (anhydrous), methanol and 1,2-
dichloroethane (DCE) were obtained from National Medicines
Corporation Ltd. of China, all of which were of analytical grade
and were used as received. Chloromethyl Polystyrene Resin
(CPR, Aladdin, 100-200 mesh), formaldehyde dimethyl acetal
(FDA, Alfa Aesar, 98%) and 1,3,5-triphenylbenzene (Alfa Aesar,
98%) were also used as received.
Preparation of catalysts
In the experiments, KTB (KB) powders as the initial
precursors were prepared according to our previous report20.
The C-(KTB-Pd) catalyst was prepared using an impregnation
method. In a typical synthesis, 10 mg PdCl2 was dissolved in 2 ml
acetonitrile at a flask, after the solution changed to bright yellow,
80 mg of KTB was dispersed in it and mixing for 12h, the
suspension was centrifugal separation and washed by acetone
for three times. The slurry was dried in an oven at 60°C for 6h,
than the resulting powder was heated in a tube furnace at 500°C
under flowing N2 for 4 h, the heating rate is 2oC/min and then
Previously we have proposed
a low-cost strategy to
synthesize high surface area microporous knitting aryl network
polymers(KAPs) via one-step ‘knitting’ of rigid aromatic
compounds blocks with an external crosslinker20. Herein we
describe the first preparation of novel dual-porous carbon-
supported Pd NP catalysts via the carbonization of KAPs-1,3,5-
triphenylbenzene (C-(KTB-Pd)), KAPs-benzene (C-(KB-Pd)) and
Chloromethyl Polystyrene Resin(C-(CPR-Pd)) (Scheme 1, see
Supporting Information(SI) for experiment details). Pd NPs
were generated in these three porous materials, by controlled
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reduction at 400°C under an H2 atmosphere for 2h. At last the
Microporous polymers prepared from different monomers
sample was cooled to room temperature at a N2 atmosphere. have a remarkable difference in their surface area and rigid
The applied calcination temperatures for C-(KTB-Pd) were structure. When 1,3,5-triphenylbenzene was used as
a
varied from 500 to 800 °C for 4h, and then reduction at 400°C monomer, the rigidity of the obtained network was far
under an H2 atmosphere for 2h. The catalysts C-(KB-Pd) and C- stronger than that prepared using benzene as a monomer. As
(CPR-Pd) were also prepared following the same procedure, shown in Table 1 (entry 2), the surface area of KB is 1351 m2g-1,
the detals was present in supporting information (ESI).
but the ratio of micropores is only 19.8 %, KTB has a 62 %
micropore but its surface area is 1059 m2g-1. The CPR has
almost no micropores and its surface area is 24m2g-1. The
presence of the micropores is very useful to contain and
immobilize smaller Pd NPs. The Pd NPs are confined in the
Table 1. The catalysts properties and activity comparison of
three catalysts
Characterization
Transmission electron microscopy (TEM) was carried out
using a FEI Tecnai G2 20 electron microscope running at 200
kV. The average Pd NPs size was calculated by counting over
200 particles. The XRD patterns were obtained on a Bruker
Advanced D8 diffractometer over a 2θ range of 5-90° with Cu
Kα radiation. Pd contents were analyzed by atomic absorption
a
Entry Sample
SBET
m2/g
MPb Pd
average
Yieldd
%
spectrometry (AAS) on
a Perkin Elmer AA-300. X-ray
%
wt% diameterc
nm
photoelectron spectroscopic (XPS) analysis was carried out on
a VG Multilab 2000 spectrometer using Al Kα radiation at a
power of 300 W. The pass energy was set at 100 eV, and C1s
line at 284.6 eV was used as a reference. All the samples were
washed with acetone and dried before XPS characterization.
The surface area, N2 adsorption isotherms (77.3K), and pore
size distribution of prepared catalysts were measured using
Micromeritics ASAP 2020M surface area and porosity analyzer.
Before analysis, the samples were degassed at 110 °C for 8 h
under vacuum (10−5bar). The products of the coupling reaction
were identified by 1H NMR spectra using a 400 MHz Bruker
AV400 instrument in CDCl3. Chemical shifts are reported in
parts per million (ppm) downfield from TMS.
1
2
3
KTB
KB
1059
1351
24
62.0
-
-
-
-
-
-
-
-
-
19.8
7.9
CPR
4
5
C-(KTB-Pd)
C-(KB-Pd)
924
775
58.3 1.1
4
7
96
92
21.4 0.98
The Heck coupling reaction
6
C-(CPR-Pd) 439
31.9 0.81
16
62
All reactants and solvents (Aldrich/Fluka) were used without
further purification or drying. Typically, the Heck reaction was
carried out as follows: iodobenzene (1mmol), trihydrate
potassium phosphate (1.5 mmol), and styrene (1.2 mmol)
were dissolved in dimethyl formamide (DMF) (2ml) contained
in a round bottom flask (10 ml) equipped with a reflux
condenser. The flask containing the reaction mixture was
immersed into an oil bath at a temperature of 120 °C under
vigorous stirring for a certain period to preheat the reaction
mixture, and then catalyst was added into liquid mixture to
initiate the reaction. After the reaction end, the mixture was
centrifugal filtration to separate the liquid phase. The product
was obtained by preparative TLC using a mixed solution of
ethyl acetate and petroleum ether (1:10, v/v) as eluting
solvent. The products were characterized and confirmed by
1HNMR.
aSurface area calculated from N2 adsorption isotherms at 77.3K
b
using BET equation. Microporous proportion calculated from
c
Pore volume and Micropore volume. average diameter was
counted over 200 particles in different areas of the sample
were counted. dIsolated yield. Reaction conditons:
iodobenzene, styrene, K3PO4·3H2O, 10 mg catalysts, DMF 2ml,
T=120oC and reaction time 2 hours.
micropores cavities and cannot diffuse out due to their size
limitation to cross the micropores. But MOPs with meso-
/macro structure are less appropriate candidates to act as
hosts because the Pd NPs can freely move through the
channels and eventually can interact with other Pd NPs to
grow further or migrate out of the porous supports.
For the recycling experiments, 20mg C-(KTB-Pd), 1mmol
iodobenzene, 1.2 mmol styrene and 1.5 mmol K3PO4·3H2O
were added to 2 ml DMF. The reaction was carried out at
120 °C in air for 2 h. Then the mixture was centrifugal filtration
to separated. The obtained catalyst was washed by distilled
water and acetone at least three times, dried under vacuum
and re-used in the next run.
After carbonization at a certain temperature (500°C), the
surface area of three catalysts changed to 775 and 924 m2g-1
respectively (Table 1, entry 4-5). The degree of the decreased
surface area obviously complies with the rigid network and
microporous structure of these materials. So the Pd NPs also
couldn’t grow larger even during carbonization due to their
confinement in the micropores of these materials
carbonization. However, the surface area of C-(CPR-Pd) has
increased to 439 m2g-1, we considered the Chloromethyl
Results and Discussion
Catalysts characterization
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Polystyrene Resin (CPR) is rich in macroporous, when 4). In addition, the temperature (Table 2. entry 1-4) and pore
carbonization at high temperature, the collapse and structure (Figure 2(b)) has remarkable impact on the
decomposing of part structure result in abundant micro/ meso dispersion and size of Pd NPs, the highest dispersion and the
pore greatly increase the surface area.21
smallest size is obtained at 500 °C.
The porous properties of three catalysts were analysed by
N2 sorption analysis. As shown in Figure 2(a), the adsorption
isotherms of all three catalysts indicate a steep N2 gas uptake
at low relative pressure (P/P0< 0.001), thus indicating an
extensive micropore structure. The flat course in the
intermediated section is a typical type I feature according to
the IUPAC classification. At higher relative pressures, however,
the isotherms for the three catalysts differed quite markedly.
The N2 isotherm for C-(KTB-Pd) is very similar to our previously
TEM images show a uniform size distribution of Pd NPs with
near spherical morphology encaged in the novel micro carbon
materials (Figure 1), with an average diameter of 4, 7, 16 nm
for C-(KTB-Pd), C-(KB-Pd), C-(CPR-Pd), respectively. It was also
observed that C-(KB-Pd) contains uniform and smaller Pd NPs
than those generated in C-(CPR-Pd) (Figure 1 e, f). Moreover,
the average size of palladium particles can be tuned in the
range of 4-6 nm (Figure1 d) on C-(KTB-Pd). The results agree
with the greater microporosity of C-(KTB-Pd), and the
protection of microsporous and rigid network. These
properties prevent the collapse of catalyst and keep the Pd
NPs apart with a better dispersion and smaller size. The results
demonstrate that the pore structure has a strong influence on
the growth of Pd NPs during carbonization, the smaller,
narrower pore distribution and strong rigid network leads to
the generation of smaller Pd NPs.
reported catalyst20 and can be classified as Type
Ⅰ. The
isotherm of C-(CPR-Pd) exhibits more Type II character and
displays pronounced pore filling at high relative pressures,
which is in consistent with the presence of larger pores that
allow unrestricted multilayer adsorption.23 The isotherm for C-
(TB-Pd) can also be classified as Type II but with evidence of
Some Type IV character, only a small degree of hysteresis was
Table 2. The catalytic activity comparison of different
carbonization temperature
a
Entry Temperature SBET
Average particles(nm) Yield
(%)d
m2/g
HRTEM b
XRDc
°C
1
2
3
4
500
924
810
722
713
4.0
7.6
96
600
700
800
10.3
17.6
48.7
16.2
25.4
45.6
90
60
17
aSurface area calculated from N2 adsorption isotherms at 77.3
K using BET equation. b Over 200 particles in different areas of
the sample were counted. cCalaulated from the Scherrer
Formula. dIsolated yield. Reaction conditons: iodobenzene,
styrene, K3PO4·3H2O, 10 mg catalyst, DMF, T=120 °C and t = 2h.
Figure 1. TEM images and the particle size distribution of Pd
nanoparticles supported on three microporous carbon. a) C-
(KTB-Pd). b) C-(KB-Pd). c) C-(CPR-Pd)
The influence of temperature on the Pd NPs size is shown
in Figure S1 (ESI). The picture
a clearly shows the
encapsulation of Pd NPs by porous carbon, and b, c, d show
the different Pd NPs size of the sample C-(KTB-Pd) at different
carbonization temperature. During the use of NPs embedded
inside porous materials as heterogeneous catalysts, one of the
key points to be addressed is their stability. Compared to
MOFs, MOPs have a better thermal stability. When the
calcination temperature is increased from 500 to 800 °C, the
average diameter of Pd NPs is increased from 4 to about 50
nm, and the surface area is reduced to 713m2g-1 (Table 2. Entry
Figure 2. (a) Nitrogen adsorption isotherm and desorption
isotherm at 77.3 K. (b) pore size distribution of three catalysts.
observed upon desorption, indicating the presence of
mesopores and macropores. From the pore size distribution
data (Figure 2(b)), we conclude that the microporosity and
rigid structures are in favor of the formation of small Pd NPs.
The catalyst C-(KB-Pd) and C-(CPR-Pd) has some micropores,
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but compared to the mesopores and macropores, it is very a average diameter of 4-6 nm, while the lowest yield was
small proportion. The average pore size of C-(KTB-Pd), C-(KB- obtained with palladium particles of about 50 nm diameters
Pd), C-(CPR-Pd) are 2.6, 4.9 and 4.5 nm respectivily, calculated (Table 2, entry 4).
from 4VA−1 and the pore size distribution of catalysts were
calculated using DFT methods.
Table 3 The effect of solvents and basea
The high angle XRD patterns of the Pd NPs supported on
microporous carbon with different carbonization temperature
are shown in Figure 3. The typical diffraction pattern of
palladium fcc crystalline structure is clearly observed. The
Entry
Solvent
Base
T (°C)
Yield (%)b
diffraction peaks become sharper and more intense with the
increased temperature, suggesting an increased crystalline
domain size, which is in consistent with the TEM results (Table
2). The increasing crystalline domain size is due to the
enhanced aggregation of palladium clusters or particles at a
higher temperature (800 °C) during carbonization process.
1
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMAc
H2O
K3PO4·3H2O
K3PO4·3H2O
K3PO4·3H2O
Na2CO3
120
100
80
96
74
55
37
95
12
94
62
93
94
32
88
85
12
74
89
73
92
78
2
3
4
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
5
EtONa
6
Cs2CO3
7
KOt-Bu
8
KOAc
9
NaHCO3
10
11
12
13
14
15
16
17
18
19
NaOAc·3H2O
K2HPO4
K2CO3
K3PO4·3H2O
K3PO4·3H2O
K3PO4·3H2O
K3PO4·3H2O
EtOH
Toluene
cyclothexane K3PO4·3H2O
Figure 3. High angle XRD patterns of the Palladium support
microporous carbon C-(KTB-Pd) at different carbonization
temperature. a) 500°C, b) 600 °C, c) 700 °C, d) 800 °C.
DMSO
NEt3
K3PO4·3H2O
K3PO4·3H2O
20c
21d
DMF
DMF
K3PO4·3H2O
K3PO4·3H2O
120
120
95
95(76)
1 mmol of
In order to get an insight into oxidation state of palladium in
C-(KTB-Pd), X-ray photoelectron spectroscopy (XPS)
experiments were performed (Figure S2 ESI). The result
displays the binding energy of Pd 3d for C-(KTB-Pd). It exhibits
two observable peaks centred at 335.75 and 341.05 eV, which
are assigned to Pd 3d5/2 and Pd 3d3/2, respectively. This result
implies zero oxidation state of Pd.
aReaction conditons: 10 mg of C-(KTB-Pd),
iodobenzene, 1.2 mmol of styrene, 1.5 mmol of base, 2 ml of
solvent, 120 °C, 2h in air. bIsolated yield. cN2 atmosphere.
dCommercial 5% Pd/C as catalyst, and the yield in parentheses
is the second runs.
Based on the above results, we successfully tuned the size of
Pd NPs in the range of 4-20 nm on porous carbon supports.
Moreover, the average size of palladium particles can be
controlled at 4 nm by controlling the porous structure and
calcination temperature.
In order to evaluate the catalytic performance of C-(KTB-Pd),
the Heck reaction of aryl halides and olefins were studied. First,
we examined the activity of C-(KTB-Pd), C-(KB-Pd), C-(CPR-Pd).
As shown in Table 1, using 0.1mol % Pd with respect to
o
iodobenzene, C-(KTB-Pd) gave a 96% yield within 2h at 120 C
Catalytic activity of catalyst C-(CKB-Pd)
(Table 1, entry 4), showing obvious advantage over C-(KB-Pd)
and C-(CPR-Pd) in the controlled experiments (Table 1, entry
5,yield 92% and enrty 6, yield 62% ). Table 2 (entry 1-4)
illustrates that the higher carbonization temperature results in
the production of larger Pd NPs. The smallest Pd NPs gave the
highest yield (Table 1, entry 1). So we selected the C-(KTB-Pd)
carbonization at 500°C as the catalyst for the reactions
reported in this article. The influence of temperature, base,
and solvent on catalytic property of C-(KTB-Pd) was
systematically investigated by using the coupling of
The Heck reaction has been proved to be structure sensitive
when catalysed by metal nanoparticles based catalysts. The
trend is attributed to the fact that smaller Pd NPs possess a
higher catalytic efficiency. In the present study, the size of C-
(KTB-Pd) supported Pd NPs is varied by applying different
carbonization temperature and controlling the pore diameter
distribution of the support. The results are shown in Table 1
and Table 2 indicating that smaller Pd NPs are more active to
get the desired products in Heck coupling reactions. The
product yield increases with a decrease in the size of the Pd
NPs. The highest yield was obtained with Pd NPs with an
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iodobenzene and styrene as a model reaction. The results are increased from 55% to 96% (Table 3, entries 1-3) and the
summarized in Table 3.
highest catalytic activity was achieved at 120°C (Table 3, entry
1). The effect of base is shown in Table 3 (Entries 4-12) and the
result shows the stronger base gave the higher yield, Table 3
(Entries 5, 7, 10), and K3PO4·3H2O is found to be the best base
for this purpose. The solvent used in the Heck reaction also has
a great influence on theC-(KTB-Pd) activity, as shown in Table 3
(Entry 13-19). It is observed that the suitable solvent is N, N-
Dimethyl formamide (DMF) and Dimethyl sulfoxide (DMSO)
resulting in a 96% and 92% yield, respectively (Table 3,
entries1 and 18). In water, ethanol and cyclohexane (Table 3,
entry14-15, 17), the catalytic activity is low. More interestingly,
the change in the reaction environment from inert to ambient
had no negative effect on activity of C-(KTB-Pd) activity (Table
3, entry20). Generally, homogeneous catalysts (usually with P
and N ligands) used in the Heck reactions are very sensitive to
the presence of air during the reaction,22 C-(KTB-Pd) catalyst
would be more suitable for practical applications of such
catalysts. We also compared the catalytic efficiency with
commercial Pd/C (5% wt) catalyst. Thought it give high yield in
first run (table 3, entry 21), the yield decrease to 76% in
second runs.
Table 4. The Heck coupling reaction of different aryl halides with
different alkenes over C-(KTB-Pd)a
R1
C-(KTB-Pd base)
R
R
I
R1
DMF 120 o
C
R1 = H, CH3, n-Bu,Cl
Entry
Aryl halide
Product
Time
(h)
Yield
(%)b
99
1
2
3
4
5
6
7
8
9
4
2
4
3
4
4
4
3
4
99
93
99
96
95
99
96
80
Reactivity for various substrates of catalyst C-(KTB-Pd)
To examine the scope for utilization of C-(KTB-Pd) in the
Heck reaction, a variety of aryl halides were coupled with
different olefins in DMF in the presence of 10 mg C-(KTB-Pd)
using K3PO4·3H2O as a base. The results are summarized in
Table 4. For iodobenzene and other aryl iodides with
substituents such as –CH3 and –OCH3, good yields (80-90%) of
the products were observed within 2-4 h (Table 4, entries 1–
11). Further study indicated that aryl iodides with larger steric
bulk also shows a high activity with aryl styrene (Table 4,
entrys12-17). Aryl bromides are difficult to be activated
compared to aryl iodide, probably because of stronger C-Br
bond. As a result, the coupling reactions of aryl bromides with
different olefins required higher temperature and extended
reaction times even in the presence carbon supported Pd NPs
(Table 4, entry 17-18).
10
4
83
11
12
4
2
97
94
13
14
15
16
4
4
4
4
97
97
92
90
We also adopted a hot filtration experiment to determine if
the heterogeneous or the dissolved homogeneous Pd species
were the real catalysts responsible for the reaction between
iodobenzene and styrene. After 30 minutes, we stopped the
reaction immediately and remove the solid catalyst from the
reaction mixture. The yield obtained was 38%. The mother
liquor was allowed to react for another 90 minutes under the
same conditions but the final yield was 38.6%, no obvious
increase in yield was observed, and the Pd content of the
mother liquor was 2.2 ppm according to AAS. The results
indicate that there only little Palladium species leached and
the active phase was not the dissolved palladium species
leached from the supports.
17c
18c
12
12
77
72
Recycle performance comparison of three catalysts.
aReaction conditions: 10 mg of C-(KTB-Pd), 1 mmol of
iodobenzene, 1.2 mmol of styrene, 1.5 mmol of base, 2 ml of
solvent, 120 oC in air. bIsolated yield in parentheses. c 140 °C.
The reaction temperature has a significant effect on the C-
(KTB-Pd) activity. With increasing reaction temperature, yield
Another important issue concerning the use of a solid
catalyst is its reusability and stability under reaction conditions.
To gain insight into this issue, the recycling studies were
performed for the Heck coupling reaction, by regenerating and
then reusing the catalyst under the same conditions (Figure 4).
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Before reuse, the solid was separated by filtration and
washed with water and acetone several times. Almost similar
products yield (95%) was achieved after 10 recycling
experiments of catalyst C-(CTB-Pd), and the TEM images
(Figure S3 ESI) of the reused catalyst clearly show no
significant change in the size of Pd NPs on porous carbon
supports after the reaction. These results thus illustrate the
high stability and excellent reusability of the catalyst. We also
take kinetic study to research the recycle performance of
catalyst C-(KTB-Pd), (Fig S4 ESI) the result review the catalyst
run 5 also give a good yield in 2 hours, probably we not rely on
expend reaction time to improve the yield.
financially supported by National Natural Science Foundation
of China (No. 21473064/ 21474033/ 51273074)
Notes and references
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styrene, 1.5 mmol of K3PO4·3H2O, 2 ml DMF, 120 oC, 2h in air.
Conclusions
The study encompasses the first description and simple
preparation of C-(KTB-Pd) catalysts through the method of
direct carbonization and reduction. The prepared palladium
nanoparticles were in the form of near spherical particles of
<6nm diameter, the size being adjustable by changing the
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High dispersed C-(KTB-Pd) catalyst not only has high activity for
Heck coupling reactions, but also offers many promising
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SYNOPSIS TOC
A highly dispersed Palladium nanoparticles encaged by micro
carbon and exhibit excellent cycling performance towards
Heck cross-coupling reaction
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